An elastomer seal between a piston and a cylinder or a shaft of an automatic automobile transmission.
The instant invention relates to an elastomer seal between a piston and a cylinder or a shaft of an automatic automobile transmission, the piston and cylinder or shaft carrying out translatory or linear movement with respect to each other, when in operation.
Fluid operated pistons in the mechanical portion of automatic motor vehicle transmissions serve to shift multiple disc clutches or brake bands. Such pistons are exposed to high fluid operating pressures of up to 20 bars, the fluid usually being an hydraulic fluid. Failure of such a piston normally results in failure of the entire transmission. For this reason the component parts of the piston, including the elastomer seal between the piston and a cylinder or shaft along which the piston travels must meet demands of very high reliability.
It is known to join an elastomer seal for a piston of the kind described to the support body of the piston by vulcanizing. Pistons of such structure are referred to as xe2x80x9ccompound pistonsxe2x80x9d. An example of a compound piston of this kind is illustrated in FIG. 1. It shows an elastomer seal 1, including a sealing lip 2 which extends around a gap (c) to be sealed against a sealing surface 10, the seal being vulcanized to a circumferential surface 3 which faces the cylinder or shaft. Both sides 13, 14 of the support body 11 are framed by radial flanges 4, 5 of the elastomer seal, while a web 12 which interconnects the flanges covers the periphery 15 and thus partly fills the gap (c) towards the sealing surface 10. During the vulcanizing process the elastomer flows, adopting the U-shaped configuration depicted in FIG. 1. In reality, the dimension of the gap (c) is approximately 1 mm.
The piston is moved in the direction of arrow f by hydraulic pressure p exerted on the right side of the compound piston 1, while atmosperic pressure prevails at the left side, as see in FIG. 1. The pressure p raises the contact pressure of the sealing lip 2 against the sealing surface 10 and, at the same time, generates a thrust or shear force directed radially inwardly at the adhesion joint between the leg 5 of the elastomer seal 1 and the sidewall 14, the extent of this shear force being great, in dependence on the gap width (c). If a permissible shear force is exceeded, the adhesive bond will be broken, with resulting failure of the elastomer seal and thus of the compound piston.
An elastomer seal for an annular piston of an hydraulic clutch disengaging device, which piston is movable linearly on a guide sleeve, is described in German patent application laid open DE 196 14 385 A1. A clearance between the annular piston and the guide sleeve is neither defined in the specification nor indicated in the drawings.
It is the object of the instant invention to design an elastomer seal of the kind defined initially such that the shear forces in the adhesion joint between the elastomer seal and the supporting body are minimized. It is also an object of the invention to obtain the joining of the elastomer seal to the piston by an automated inexpensive manufacturing process.
Claim 1 serves to meet those objects.
Advantageous modifications are protected by the subclaims. In a particularly advantageous further development a bead formed at the end of the elastomer seal remote from the lip is seated in a groove in the circumference of the supporting body and likewise adhered to the same. This presents a combination of the advantages of a normal seal form-locked in a groove and those of a bond obtained by vulcanizing while using an adhesive. The shearing stress acting in the adhesion joint is reduced to minimum values because the axial force generated by the operating pressure and acting on the sealing lip is transmitted almost entirely directly to the supporting body. Examinations of embodiments as defined in claim 4 have revealed that the shear forces in the adhesion joint are reduce to less than one tenth of the shear forces acting in the adhesion joint of the known embodiment as illustrated in FIG. 1.
It is pointed out expressly that an elastomer seal according to the invention is useful both with solid piston designs and with annular pistons. In the latter case, a seal according to the invention is provided not only at the inner diameter but also at the outer diameter, the seal at the inner diameter acting with respect to a central shaft and the seal at the outer diameter acting with respect to the inside wall of a cylinder.